My artist is primarily familiar with photoshop, and wished to use it as her primary tool for creating art for the NES. In addition, she feels much more comfortable with a pure 24 bit, RGB format than with an indexed color format. This produced an interesting challenge for me when writing my own personal screen tool (similar to shiru's tool, but specific to my needs). What seems to happen is that, even though one may have turned off all anti aliasing features of photoshop, every now and then it screws up and changes an RGB value slightly between adjacent pixels. Even if she picks from a constant set of colors this seems to happen. So, my screen tool has the capability of detecting small differences in color and "flattening" all such pixels to the same color. After this is done, it makes sure that what the artist has drawn is displayable on the NES (correct use of attributes, does not exceed 13 colors, etc.). If there are any errors at this point, it generates "error report" bitmaps with bright markers around the problem squares (16x16 squares for bg, 8x8 for sprites). This has been immensely helpful when working with an artist who is not interested in learning a non-standard tool only available in our community.

I was curious if there are similar error correcting/reporting bitmap importing tools out there, or if anyone else has a similar situation with their artist. If not, hope it was interesting anyway

Using Photoshop for pixel art doesn't make much sense, since its strong point is image filtering and processing, things you absolutely don't need in this case. But since that's the only thing your artist is going to use, it's your job to adapt everything. I wouldn't worry about programming my own conversion code though, that seems like a huge waste of time.

I think the first step would be to give the artist a palette, so that only those colors are used. Like you said, there will probably be slight color variations due to the way Photoshop filters images when scaling them and such, but as long as you have the original palette, you can quickly convert the image to indexed mode and select the clean palette, so that Photoshop itself will remap the similar colors to the one they should all be.

Other than that, I can't think of any other kind of automatic processing you could do. Ideally your artist is aware of the system limitations and will draw accordingly, but if that's not the case you will have to adapt everything yourself, which turns your artist's actual title to "conceptual artist", seeing as you will have to use their work as a base for the final product.

I faced some of that back when I asked my wife (girlfriend back then) to draw me some sprites. I explained the basic limitations, but there was no way she would grasp all of it, so I always had to move some things around, slightly reduce the color count, things like that.

I've done something similar in previous projects, though with an added step. IMHO when you convert something from a high color palette to something limited like NES's it's best to hand convert it. GIMP/Photoshop is just not intuitive enough to convert this, it needs some human touch. For example:

Let's say I want to convert this image to something the NES can use:

If I convert it to NES's palette it looks pretty ugly (to me) and there's too many colors for the NES to handle:

I've narrowed my palette down to 4 colors, and I try to convert it again, but still not even close to what I want:

Then, in this final image I hand drew the pixels over the original art (image) and I'm happy with the result. (Though on a real NES on a CRT the dithering would probably look bad )

EDIT: If you want I can take a piece of your art and hand convert it in a [private if you wish] video as an example.

I may have described our process inadequately. My artist starts out drawing to NES color constraints. It is already as close as she can get it to "being a screen shot the NES can display." What my importer does is correct small anti aliasing errors that photoshop introduces despite having turned off all anti-aliasing. This allows her to have it be "close enough" and not cause us too many problems, thus working around her lack of desire to learn a new tool

What seems to happen, for example, is say she's using black, RGB: 1, 1, 1. Sometimes, even with anti aliasing turned off, photoshop will write 0, 0, 0 to an adjacent pixel instead. My program will interpret both 1, 1, 1 and 0, 0, 0 as the same color because they are similar to within a certain percentage. This seems to work well since the NES only has a small number of colors---most of the errors that have occurred for us are well within the difference of colors in the real NES palette. So, she can simply use a bitmap of the NES master palette while drawing, and our importer will match colors as best it can. The results usually turn out pretty good...though I recognize this is rather quirky and probably won't be useful to more than a very small minority of nes devrs out there. I thought I'd find out though =)

Cartlemmy, that's pretty neat. My avatar is also me but not in 8 bit style. It'd be neat to see what that looks like 8 bit style, but I'd feel like I was copying you =D

Really!? To me it looks like an automatic color reduction that would take around 1 minute to make, upload AND post. cartlemmy's actually looks like it used a lot of manual adjustments.

10 minutes from seeing the thread to appearing on imgur.com.
There were some adjustments made from a simple color reduction. I like the method where you reduce the colors, THEN remap the palette. Only problem is that Photoimpact's "Color on screen" feature broke in Windows XP, so it takes longer to assign the palette colors.

I actually reduced this to 5 colors, then merged two colors together.

Things I don't like about Photoimpact is that nearest neighbor resizing is broken (need to screen capture Zoom mode!), and the Color On Screen feature for building palettes is broken, so it takes way too long to build palettes.

I may have described our process inadequately. My artist starts out drawing to NES color constraints. It is already as close as she can get it to "being a screen shot the NES can display." What my importer does is correct small anti aliasing errors that photoshop introduces despite having turned off all anti-aliasing. This allows her to have it be "close enough" and not cause us too many problems, thus working around her lack of desire to learn a new tool

What seems to happen, for example, is say she's using black, RGB: 1, 1, 1. Sometimes, even with anti aliasing turned off, photoshop will write 0, 0, 0 to an adjacent pixel instead. My program will interpret both 1, 1, 1 and 0, 0, 0 as the same color because they are similar to within a certain percentage. This seems to work well since the NES only has a small number of colors---most of the errors that have occurred for us are well within the difference of colors in the real NES palette. So, she can simply use a bitmap of the NES master palette while drawing, and our importer will match colors as best it can. The results usually turn out pretty good...though I recognize this is rather quirky and probably won't be useful to more than a very small minority of nes devrs out there. I thought I'd find out though =)

Cartlemmy, that's pretty neat. My avatar is also me but not in 8 bit style. It'd be neat to see what that looks like 8 bit style, but I'd feel like I was copying you =D

Oh man, I know what you're talking about with Photoshop and I hate it. I mean, I like Photoshop in general, but not for pixel art purposes! I forget when I first learned about this automatic color changing even when you've disabled everything that ought to do that...I think I was editing some screen shots, and every time I used a brush or fill bucket I was inadvertently changing all the colors. Finally I compared two sprites side by side and noticed that one was visibly lighter than the other. That was when I switched to MSPaint and more accurate programs like GraphicsGALE. Why in the world would you want colors to change like that?

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